Prior AM transmitters have typically employed linear radio-frequency (RF) amplifiers. However, linear RF amplifiers usually have lower efficiency (i.e., the RF output power divided by the DC input power) than that of the non-linear saturating-type RF amplifiers used in frequency-modulation (FM) transmitters. Low amplifier efficiency creates a serious detriment in portable radios, because of their limited battery capacity.
Prior amplitude modulators have used several techniques for increasing efficiency, such as the Doherty amplifier, the outphasing amplifier, and "envelope elimination and restoration" (EER). Other prior amplitude modulators have used several techniques for increasing efficiency. First, some class A modulators employ a series pass regulator to drop the DC supply voltage for the last RF stage. This circuit is simple but it has a low efficiency since most of the input DC power is dissipated in the series pass transistor.
Second, class B modulators, commonly used in AM broadcast transmitters, modulate the DC supply voltage through a modulation transformer. Generally, this practice is applicable only to full carrier voice operation, and has moderate efficiency.
Third, the switching modulator is essentially a switchingtype series pass regulator. High efficiency is possible with this type of modulator but extensive filtering is required to remove switching frequency components from the output. Such output filtering causes a poor transient response making this type of modulator unsuitable for many digital transmission applications.